BIOLOGY AND CONTROL OF CHIGGERS. 13 



made to discover and work out the development of the causative or- 

 ganism, but to no avail. 



Among the various substances that have been employed in medica- 

 tion in connection with the disease the following have been used with 

 negative results: Quinine, iodine, quicksilver, arsenics, and staining 

 preparations. From the beginning to the subsidence of the fever 

 salvarsan and trypan red have been used with very poor results. 

 An attempt has been made experimentally to utilize a serum for the 

 disease, but without results. 



As chiggers are parasitic only in their larva stage and do not 

 change hosts, it appears that the causative organisms must be trans- 

 mitted from larva to nymph, to adult, thence to egg and to larva 

 again. Such a development, although a little unusual, already has 

 a near parallel in the case of the protozoan Piroplasma bigeminum, 

 the organism of Texas fever, which is transmitted from mother to 

 Qgg to larva or to nymph, in its alternate host, the North American 

 fever tick, Margaropus annulatus Sa}^. 



In view of what is already known in regard to the transmission of 

 river fever, the biology of the chigger mites, and the general symp- 

 toms following their serious attacks on man and domestic animals, 

 the writer how predicts that in the next 50 years other serious dis- 

 eases will be shown to be transmitted by these acarids. Should these 

 mites become the transmitters of fatal diseases of domestic animals 

 on a large scale it would be found that the protection of cattle or 

 sheep from them would present a very difficult problem, as the mites 

 are so minute and so widely distributed in woodlands and along water 



courses. 



CONTROL. 



In the case of man much protection can be had from chigger at- 

 tacks by properly clothing the lower extremities or by the application 

 of repellents either directly on the skin or on the under garments. 



PROTECTION AGAINST CHIGGER ATTACK. 



Since the unengorged larvae are not over 150fjt. in width, it is seen 

 that they can pass through the mesh of many kinds of garments; 

 it is easy, however, to wear those of a weave tight enough to prohibit 

 the larvae from passing directly through the cloth. The employment 

 of tightly woven cloth, or other materials which are impervious to 

 the larvae, nevertheless, is not enough. These garments must be worn 

 so as to fit tightly around the edges or the larvae will yet have an 

 avenue of entry. 



It was frequently noticed that half-shoes exposed the ankles, and 

 for that matter indirectly the whole body, to much more serious 



4 The control of chiggers affecting poultry is considered in Farmers' Bulletin 801. The 

 measures given in the present bulletin have reference more particularly to chiggers as 

 parasites of man. 



